Power assist device and its control method

ABSTRACT

It is possible to provide a power assist device which can maintain a stable contact state without causing an oscillation phenomenon even if a robot is brought into contact with an environment. A method for controlling the power assist device is also provided. The power assist device includes: an inner force sensor which detects an operation force applied by an operator; an operation handle having the inner force sensor; a robot arm which supports the operation handle; an actuator which drives the robot arm; the actuator and a control device which measure or estimate a force applied when the robot arm is brought into contact with an environment; and the actuator and the control device which detect or estimate a motion speed of the operation handle. The control device acquires a corrected external force according to the operation force detected by the inner force sensor and an external force detected by the actuator and the control device as external force derivation means and controls the actuator so that the corrected external force acts on the operation handle.

This is a 371 national phase application of PCT/JP2008/066212 filed 9Sep. 2008, which claims priority to Japanese Patent Application No.2007-237034 filed 12 Sep. 2007, the contents of which are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a power assist device and its controlmethod.

BACKGROUND OF THE INVENTION

Conventionally, at the scene of manufacture, a robot referred to as apower assist device is used so as to reduce labor and improveworkability. As a control method of the power assist device, mechanicalimpedance control based on a position control (called an admittancecontrol) is widely adopted.

In the admittance control, normally, position control with a high gainis performed and its target position is determined based on a forcesensor. Then, force applied on the power assist device not through theforce sensor is not considered at the control action, and a manipulatoris hardly actuated. Namely, when the power assist device is brought intocontact with an environment (the external) at the position except thedetection part of the force sensor, the power assist device is continuedto be actuated without consideration of the contact.

Hereafter, situations where humans and robots share a space willincrease more and more so that the admittance control will consider theconnection of a robot with a person or an environment

The Non-patent Literature 1 shown below discloses a method of theadmittance control with consideration of connection with an environmentby using estimated value of external force acting on the power assistdevice not via any force sensor.

In the control method with consideration of the connection with theenvironment disclosed in the Non-patent Literature 1, compliant actionfollowing unknown external force can be realized. However, in thiscontrol method, there is known that, when an operator brings the robotinto contact with the environment and then presses the robot to theenvironment further, the robot vibrates along the pressing direction andthe anti-pressing direction (so-called oscillation phenomenon occurs).Essentially, the oscillation phenomenon should not occur in the statethat the operation force is perfectly balanced with the external force.However, even slight difference between the two causes the oscillationphenomenon. The oscillation phenomenon becomes a problem in the casethat the connection of the robot with the environment is premised suchas pressing work with a power assist device.

For solving the problem, hitherto, such a method that an operatoradjusts pressing condition of the power assist device is only obtained.With regard to parameters concerning the action of the power assistdevice, viscosity resistance is set to be increased so as to suppressthe oscillation phenomenon. However, in this case, another problemoccurs that the operation force required for the operation of the powerassist device is increased so as to worsen the operability, whereby itis undesirable in practical use.

Namely, with regard to the conventional art, there is no methodeffective to suppress the oscillation phenomenon caused in the pressingwork with the power assist device.

[Non-patent Literature 1]: Masahito Iso, Hirokazu Seki, and Yoichi Hori,Sensorless power assist method using impedance control considering loadcharacteristics, Technical Meeting on Industrial Instrumentation andControl, IEE Japan, Vol. IIC-02-40 (2002)

SUMMARY OF INVENTION Problems to be Solved by the Invention

The purpose of the present invention is to provide a power assist devicewhich can maintain a stable contact state without causing an oscillationphenomenon even if a robot is brought into contact with an environment,and to provide a method for controlling the power assist device.

Means for Solving the Problems

The first aspect of the present invention is a power assist device whichincludes an operation part operated by an operator; an operation forcedetection means for detecting operation force applied on the operationpart; a robot arm for supporting the operation part; a drive means fordriving the robot arm; an external force measurement means for measuringforce applied when the robot arm is brought into contact with anenvironment, or an external force estimation means for estimating forceapplied when the robot arm is brought into contact with the environment;and a speed detection means for detecting movement speed of theoperation part, or a speed estimation means for estimating the movementspeed of the operation part, wherein a control device is provided thatobtains corrected external force based on the external force obtained bythe external force measurement means or the external force estimationmeans and a component along the external force of the operation forcedetected by the operation force detection means, and controls the drivemeans so as to maintain the state that the operation force and thecorrected external force act on the operation part and the componentalong the external force of the operation force and the correctedexternal force acting on the robot arm are always balanced with eachother.

In an embodiment of the power assist device according to the presentinvention, preferably, the control device compares the component alongthe external force of the operation force detected by the operationforce detection means with the external force obtained by the externalforce measurement means or the external force estimation means, regardsthe speed of the operation part calculated from the external force asspeed of the operation part along the external force calculated from thecomponent along the external force of the operation force so as toobtain the corrected external force when the component along theexternal force of the operation force is opposite to the external forceand the external force is larger than the component along the externalforce of the operation force, and adopts the external force without anyoperation as the corrected external force when the component along theexternal force of the operation force has the same direction as theexternal force or the external force is smaller than the component alongthe external force of the operation force.

In another embodiment of the power assist device of the presentinvention, preferably, the control device compares the component alongthe external force of the operation force detected by the operationforce detection means with the external force obtained by the externalforce measurement means or the external force estimation means, regardsthe external force as a component of the operation force along theexternal force so as to obtain the corrected external force when thecomponent along the external force of the operation force is opposite tothe external force and the external force is larger than the componentalong the external force of the operation force, and adopts the externalforce without any operation as the corrected external force when thecomponent along the external force of the operation force has the samedirection as the external force or the external force is smaller thanthe component along the external force of the operation force.

The second aspect of the present invention is a control method of apower assist device which includes an operation part operated by anoperator; an operation force detection means for detecting operationforce applied on the operation part; a robot arm for supporting theoperation part; a drive means for driving the robot arm; an externalforce measurement means for measuring force applied when the robot armis brought into contact with an environment, or an external forceestimation means for estimating force applied when the robot arm isbrought into contact with the environment; or an external forceestimation means for estimating force applied when the robot arm isbrought into contact with the environment; and a speed detection meansfor detecting movement speed of the operation part, or a speedestimation means for estimating the movement speed of the operationpart, wherein corrected external force is obtained based on the externalforce obtained by the external force measurement means or the externalforce estimation means and a component along the external force of theoperation force detected by the operation force detection means, and thedrive means is controlled so as to maintain the state that the operationforce and the corrected external force act on the operation part and thecomponent along the external force of the operation force and thecorrected external force acting on the robot arm are always balancedwith each other.

In an embodiment of the control method of the power assist deviceaccording to the present invention, preferably, the speed along theexternal force of the operation part calculated from the component alongthe external force of the operation force detected by the operationforce detection means is compared with the speed of the operation partcalculated from the external force obtained by the external forcemeasurement means or the external force estimation means, the speed ofthe operation part calculated from the external force is regarded as thespeed along the external force of the operation part calculated from thecomponent along the external force of the operation force so as toobtained the corrected external force when the speed along the externalforce of the operation part calculated from the component along theexternal force of the operation force is opposite to the speed of theoperation part calculated from the external force and the speed of theoperation part calculated from the external force is larger than thespeed along the external force of the operation part calculated from thecomponent along the external force of the operation force, and theexternal force without any operation is adopted as the correctedexternal force when the speed along the external force of the operationpart calculated from the component along the external force of theoperation force has the same direction as the speed of the operationpart calculated from the external force or the speed of the operationpart calculated from the external force is smaller than the speed alongthe external force of the operation part calculated from the componentalong the external force of the operation force.

In another embodiment of the control method of the power assist deviceaccording to the present invention, preferably, the component alongexternal force of the operation force detected by the operation forcedetection means is compared with the external force obtained by theexternal force measurement means or the external force estimation means,the external force is regarded as a component of the operation forcealong the external force so as to obtain the corrected external forcewhen the component along the external force of the operation force isopposite to the external force and the external force is larger than thecomponent along the external force of the operation force, and theexternal force without any operation is adopted as the correctedexternal force when the component along the external force of theoperation force has the same direction as the external force or theexternal force is smaller than the component along the external force ofthe operation force.

Effect of the Invention

According to the power assist device of the present invention, when thepower assist device is brought into contact with the environment, theoscillation phenomenon is prevented and the stable connection state ismaintained.

According to the power assist device of the present invention, insteadof correcting the operation force, the external force is corrected to belimited so as to suppress the oscillation phenomenon on the power assistdevice without spoiling the operability.

According to the power assist device of the present invention, theoscillation phenomenon on the power assist device is suppressed morecertainly without spoiling the operability.

According to the control method of the power assist device of thepresent invention, when the power assist device is brought into contactwith the environment, the oscillation phenomenon is prevented and thestable connection state is maintained.

According to the control method of the power assist device of thepresent invention, instead of correcting the operation force, theexternal force is corrected to be limited so as to suppress theoscillation phenomenon on the power assist device without spoiling theoperability.

According to the control method of the power assist device of thepresent invention, the oscillation phenomenon on the power assist deviceis suppressed more certainly without spoiling the operability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 It is a schematic drawing of state that a power assist deviceaccording to an embodiment of the present invention is brought intocontact with an environment.

FIG. 2 It is a flow chart of admittance control according to theembodiment of the present invention.

FIG. 3 It is a speed vector diagram for explaining application of thepresent invention to a multidimensional system. (a) is a first speedvector diagram, (b) is a second speed vector diagram, (c) is a thirdspeed vector diagram, and (d) is a fourth speed vector diagram.

FIG. 4 It is a schematic drawing of an experimental device forconfirming effect of application of the present invention. (a) is a planview and (b) is a side view.

FIG. 5 It is a graph of variation with time of tip position, operationforce and estimated external force at the admittance control. (a) showsan experimental result of the normal admittance control (with only theinformation of operation force), (b) shows an experimental result of theconventional admittance control (with the information of operation forceand estimated external force), and (c) shows an experimental result ofthe admittance control according to the embodiment of the presentinvention.

DETAILED DESCRIPTION

Explanation will be given on the mode for carrying out the inventionreferring appended drawings.

Firstly, explanation will be given on a power assist device 1 andcontrol method thereof according to an embodiment of the presentinvention referring work shown in FIG. 1 as an example.

As shown in FIG. 1, the power assist device 1 includes a robot arm 2, aninner force sensor 3, an operation handle 4, an actuator 5, a controldevice 6 and the like.

The robot arm 2 is supported by the actuator 5 which is a drive means,and the robot device can assume various postures by the actuation of theactuator 5 following commands from the control device 6.

The operation handle 4 which is an operation part is disposed at the tipof the robot arm 2 through the inner force sensor 3 which is anoperation force detection means, and an operator can moves the robot arm2 to a desired position by holding the operation handle 4. The operationforce applied on the operation handle 4 by the operator at the time ofthe movement is detected by the inner force sensor 3.

The actuator 5 can not only support and drive the robot arm 2 but alsodetect the posture of the robot arm 2 and detect the operation speed ofthe operation handle 4 as an operation speed detection means.

Based on the information of posture of the robot arm 2 and theinformation of operation speed of the operation handle 4 detected by theactuator 5, the control device 6 which acts as an external forceestimation means and an operation speed estimation means estimatesexternal force (contact force) applied on the robot arm 2 by anenvironment.

Though estimated value estimated by the control device 6 is adopted asthe external force applied by the environment, it may alternatively beconstructed that a force sensor which is an external force measurementmeans for connection detection is provided separately and actualmeasurement value is adopted as the external force applied by theenvironment.

When the robot arm 2 is brought into contact with the environment (forexample, an obstacle 7), operation force f_(h) by an operator andexternal force f_(e) by the obstacle 7 act on the robot arm 2. Inadmittance control, the operation force f_(h) and the external forcef_(e) are detected or estimated, and based on the detected values orestimated values, target tip position x_(dm) at which desired impedanceis realized is calculated by the control device 6. Then, the controldevice 6 controls the actuator 5 for driving the robot arm 2 so as tomove tip position x_(d) of the robot arm 2 to the target tip positionx_(dm).

In the state shown in FIG. 1, based on the information obtained byestimating (or directly measuring) the operation force f_(h) and theexternal force f_(e), a formula 1 shown below is materialized.αf _(h) +f _(e) =M{umlaut over (x)} _(d) +D{dot over (x)} _(d)  [Formula1]

In the above formula, a indicates assist ratio, M indicates inertialforce, and D indicates viscous force. In below explanation, forsimplifying the explanation, it is assumed that α=1.

In the case of calculating the target tip position x_(dm) with theinformation of a plurality of force (that is, the operation force f_(h)and the external force f_(e)) as mentioned above, the resultant of theplurality of force must be “zero” and balanced so as to set the targettip position x_(dm) to a fixed value.

Delay of force detection by the inner force sensor 3, delay caused bytime for calculation must be considered so as to obtain the target tipposition x_(dm), set to a fixed value, by the control device 6.

In an embodiment of the control method of the power assist device 1, acontrol method that the external force f_(e) used in the calculationwith the formula 1 is corrected corresponding to the state of theoperation force f_(h) and the external force f_(e) so as to prevent anoscillation phenomenon from being caused on the power assist device 1.Explanation will be given on the concrete control method.

In an one-dimensional model, in the state that the tip position x_(d)moves at speed V_(d), when the operation force f_(h) acts, a formula 2shown below is materialized and speed V_(a) is obtained.

$\begin{matrix}\begin{matrix}{V_{a} = {V_{d} + {{Tf}_{h}/M}}} \\{= {{\overset{.}{x}}_{d} + {{Tf}_{h}/{M\left( {V_{d} = {\overset{.}{x}}_{d}} \right)}}}}\end{matrix} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In the above formula, T indicates sampling time.

When the speed V_(a) and the external force f_(e) have the samedirection (that is, V_(a)*f_(e)>0), the external force f_(e) is adoptedas corrected external force f_(es) without any operation. When the speedV_(a) and the external force f_(e) have opposite directions (that is,V_(a)*f_(e)<0), the corrected external force f_(es) is obtained with aformula 3 shown below.

$\begin{matrix}{f_{es} = \left\{ \begin{matrix}{{{{- V_{a}}{M/T}} + {{DV}_{a}\mspace{14mu}\ldots\mspace{14mu}{if}\mspace{14mu}{V_{a}}}} \leq {{{f_{e} - {DV}_{a}}}{T/M}}} \\{f_{e\mspace{135mu}}\ldots\mspace{14mu}{otherwise}}\end{matrix} \right.} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Accordingly, the next target tip position x_(dm) of the operation partis obtained from the operation force f_(h) and the corrected externalforce f_(es), and the operation force f_(h) and the corrected externalforce f_(es) are maintained to be balanced with each other, whereby thecalculation result of the target tip position x_(dm) is not oscillatedand the stable connection state is maintained.

Next, explanation will be given on a control flow of admittance controlaccording to the present invention referring FIG. 2.

As shown in FIG. 2, in the control method of the power assist deviceaccording to the embodiment of the present invention, when controlaction is started, firstly, the external force f_(e) from theenvironment is measured or estimated (S01) and the operation force f_(h)from an operator is measured (S02).

Next, based on the external force f_(e) and the operation force f_(h)obtained at the steps (S01) and (S02), the control device 6 judgesconditions (S034 and S03-2), and the control flow advances to a step(S04) in the case that both the steps (S03-1) and (S03-2) are satisfiedand the control flow advances to a step (S05) in the other cases.

At the step (S04), the external force f_(e) is corrected to thecorrected external force f_(es) so as to make the next target speed“zero”. Since the speed V_(d) at the time that the robot arm 2 isbrought into contact with the environment is about “zero”, the abovecontrol can be regarded that the external force f_(e) is limited to thecorrected external force f_(es) so as to prevent vibration of the robotarm 2.

At the step (S05), the external force f_(e) is adopted as the correctedexternal force f_(es) without any operation.

Based on the corrected external force f_(es) obtained at the step (S04)or (S05), the control device 6 calculates target acceleration A_(d)(S06), and the obtained target acceleration A_(d) is temporallyintegrated so as to calculate the target speed V_(d) and the target tipposition (S07), and then the control device 6 controls the drive of theactuator 5, which supports the robot arm 2, so as to materialize thetarget speed V_(d) and the target tip position x_(dm). Each ofcalculating processes shown in FIG. 2 is performed by the control device6.

The target acceleration A_(d) is obtained with below formula 4.A _(d)=(f _(h) +f _(es) −D·V _(d))/M  [Formula 4]

The power assist device 1 includes: the inner force sensor 3 whichdetects the operation force applied by an operator; the operation handle4 having the inner force sensor 3; the robot arm 2 which supports theoperation handle 4; the actuator 5 which drives the robot arm 2; theactuator 5 and the control device 6 which measure or estimate the forceapplied when the robot arm 2 is brought into contact with anenvironment; and the actuator 5 and the control device 6 which detect orestimate the movement speed of the operation handle 4. In the controlmethod of the power assist device 1, the corrected external force f_(es)is obtained based on the operation force f_(h) detected by the innerforce sensor 3 and the external force f_(e) obtained by the actuator 5and the control device 6, and the actuator 5 is controlled so that thecorrected external force f_(es) acts on the operation handle 4.

According to the above construction, when the robot arm 2 is broughtinto contact with the environment (the obstacle 7), the oscillationphenomenon on the power assist device 1 is prevented and the stableconnection state is maintained.

The operation force f_(h) detected by the inner force sensor 3 iscompared with the external force f_(e) obtained by the actuator 5 andthe control device 6. When the external force f_(e) is larger than theoperation force f_(h), the external force f_(e) is limited to be notlarger than the operation force f_(h) so as to obtain the correctedexternal force f_(es), and when the operation force f_(h) is larger thanthe external force f_(e), the external force f_(e) is adopted withoutany operation so as to obtain the corrected external force f_(es).

Accordingly, instead of correcting the operation force f_(h), theexternal force f_(e) is corrected to be limited so as to suppress theoscillation phenomenon on the power assist device 1 without spoiling theoperability.

Furthermore, when the operation force f_(h) detected by the actuator 5and the control device 6 has opposite direction to the external forcef_(e) obtained by the actuator 5 and the control device 6 and theexternal force f_(e) is larger than a predetermined threshold, theexternal force f_(e) is limited to make the speed calculated from theoperation force f_(h) and the external force f_(e) “zero” so as toobtain the corrected external force f_(es), and when the external forcef_(e) is smaller than the predetermined threshold or has the samedirection as the operation force f_(h), the external force f_(e) isadopted without any operation as the corrected external force f_(es).

Accordingly, by setting the threshold so as to provide a dead band inthe control action, the oscillation phenomenon on the power assistdevice 1 is suppressed more certainly without spoiling the operability.

The control method of the power assist device according to theembodiment of the present invention is not limited to be adopted to aone-dimensional model and is adoptable to a multidimensional model.

Explanation will be given on the case that the control method of thepower assist device according to the embodiment of the present inventionis adopted to a multidimensional model referring FIG. 3. Herein, forsimplifying the explanation, it is assumed that viscous force D=0.

As shown in FIG. 3( a), in the state that a speed vector V_(a) by theoperation force and a speed vector Tf_(e)/M obtained from the externalforce exist as an example, the speed vector V_(a) by the operation forceobtained with the formula 2 can be indicated as FIG. 3( b) by dividingit into components (V₁, V₂) of the direction of the speed vectorTf_(e)/M obtained from the external force and the directionperpendicular thereto.

As shown in FIG. 3( c), when the speed vector Tf_(e)/M has oppositedirection to V₁ and is larger than V₁, the speed vector Tf_(e)/M iscorrected and the corrected speed vector Tf_(e)/M is referred to as −V₁.

Then, as shown in FIG. 3( d), the movement direction finally obtainedfrom the operation force and the corrected external force is V_(k).Namely, any force does not act along the direction of the external forcefrom the environment so that the oscillation is prevented.

The operation force applied along a tangent of the environment (that is,the operation of moving the power assist device along the tangent of theouter edge of the environment) is not prevented.

Next, explanation will be given on experimental results for confirmingeffect of adoption of the control method of the power assist deviceaccording to the embodiment of the present invention referring FIGS. 4and 5.

As shown in FIGS. 4( a) and (b), in the power assist device 1, the innerforce sensor 3 is disposed at the tip position of the robot arm 2, andthe operation handle 4 is disposed at the tip position of the robot arm2 through the inner force sensor 3.

An experimental device is constructed that a hook 8 a of a springbalance 8 is hanged on the operation handle 4 and an operator pulls theoperation handle 4 via the spring balance 8 so as to displace the robotarm 2.

When the operator pulls the operation handle 4 via the spring balance 8,the robot arm 2 is brought into contact with the environment (theobstacle 7), and the inner force sensor 3 detects the operation forcef_(h) at the time of the contact of the robot arm 2 with the obstacle 7.The estimated external force f_(e) by the contact and the tip positionx_(d) are obtained by calculation based on information of positionobtained from the actuator 5 driving the robot arm 2 and the like.

The experimental results with the above experimental device are shown inFIGS. 5( a), (b) and (c). In each of FIGS. 5( a), (b) and (c), the axisof abscissas indicates time (s), and a dotted line shown at the positionjust after the time 9 (s) in the graph indicates the timing of thecontact of the robot arm 2 with the environment (the obstacle 7).

Firstly, explanation will be given on the experimental result of normaladmittance control (namely, the robot arm 2 is assist-controlled withonly the information of the operation force f_(h)).

As shown in FIG. 5( a), in this case, after the contact of the robot arm2 with the environment (the obstacle 7), the tip position x_(d) and theoperation force f_(h) have been changing while the contact scarcelyaffects them.

On the other hand, the estimated external force f_(e) by the contactwhich is not used for the control changes widely so that it is obtainedthat the robot arm 2 may break the environment in this control method.

Next, explanation will be given on the experimental result of theadmittance control further considering the information of the estimatedexternal force f_(e) by the contact (namely, the robot arm 2 isassist-controlled with the information of the operation force f_(h) andthe estimated external force f_(e) by the contact).

As shown in FIG. 5( b), in this case, after the contact of the robot arm2 with the environment (the obstacle 7), the tip position x_(d) does notmake an inroad into the environment further, and it is obtained that theoscillation phenomenon occurs according to the change of the estimatedexternal force f_(e) by the contact.

In this case, though the oscillation phenomenon can be prevented byweakening the pressing force of the robot arm 2 to the environment (theobstacle 7) by the skill of the operator, this operation requiresdelicate adjustment of the force.

Finally, explanation will be given on the experimental result of theadmittance control to which the control method of the power assistdevice according to the embodiment of the present invention is adoptedso that the estimated external force f_(e) by the contact is correctedand the information of the corrected external force f_(es) isconsidered.

FIG. 5( c) shows the experimental result of the admittance controlaccording to the present invention (namely, the robot arm 2 isassist-controlled with the information of the external force f_(e)corrected corresponding to the operation force f_(h) (the correctedexternal force f_(es))).

In this case, the tip position x_(d) does not oscillate and is stable ata fixed position from the time at which the robot arm 2 is brought intocontact with the environment (the obstacle 7) to the time at which therobot arm 2 reaches the pressing state, whereby it is confirmed that thestable contact state is maintained.

Accordingly, by adopting the control method of the power assist deviceaccording to the embodiment of the present invention, even if the robotis in contact with the environment, the oscillation phenomenon does notoccur on the power assist device, whereby the power assist device can beprovided that the stable contact state is maintained at the pressingwork.

INDUSTRIAL APPLICABILITY

The power assist device and the control method thereof according to thepresent invention can be used not only for a power assist deviceprovided in an assembly line of a motorcar or the like but also forvarious uses widely in which a power assist device is used.

1. A power assist system comprising an operation part operated by anoperator; an operation force detection device that detects operationforce applied on the operation part; a robot arm that supports theoperation part; a drive mechanism that drives the robot arm; an externalforce measurement device that measures force applied when the robot armis brought into contact with an environment, or an external forceestimation device that estimates force applied when the supporting partis brought into contact with the environment; and a speed detectiondevice that detects movement speed of the operation part, or a speedestimation device that estimates the movement speed of the operationpart, wherein a control device is provided that obtains a correctedexternal force based on the external force obtained by the externalforce measurement device or the external force estimation device and acomponent along the external force of the operation force detected bythe operation force detection device, controls the drive mechanism so asto maintain the state that the operation force and the correctedexternal force act on the operation part and the component along theexternal force of the operation force and the corrected external forceacting on the robot arm are always balanced with each other, comparesthe component along the external force of the operation force detectedby the operation force detection device with the external force obtainedby the external force measurement device or the external forceestimation device, regards the speed of the operation part calculatedfrom the external force as speed of the operation part along theexternal force calculated from the component along the external force ofthe operation force so as to obtain the corrected external force whenthe component along the external force of the operation force isopposite to the external force and the external force is larger than thecomponent along the external force of the operation force, and adoptsthe external force without any operation as the corrected external forcewhen the component along the external force of the operation force hasthe same direction as the external force or the external force issmaller than the component along the external force of the operationforce.
 2. A power assist system comprising comprising an operation partoperated by an operator; an operation force detection device thatdetects operation force applied on the operation part; a robot arm thatsupports the operation part; a drive mechanism that drives the robotarm; an external force measurement device that measures force appliedwhen the robot arm is brought into contact with an environment, or anexternal force estimation device that estimates force applied when therobot arm is brought into contact with the environment; and a speeddetection device that detects movement speed of the operation part, or aspeed estimation device that estimat6es the movement speed of theoperation part, wherein a control device is provided that obtains acorrected external force based on the external force obtained by theexternal force measurement device or the external force estimationdevice and a component along the external force of the operation forcedetected by the operation force detection device, controls the drivemechanism so as to maintain the state that the operation force and thecorrected external force act on the operation part and the componentalong the external force of the operation force and the correctedexternal force acting on the robot arm are always balanced with eachother, compares the component along the external force of the operationforce detected by the operation force detection device with the externalforce obtained by the external force measurement device or the externalforce estimation device, regards the external force as a component ofthe operation force along the external force so as to obtain thecorrected external force when the component along the external force ofthe operation force is opposite to the external force and the externalforce is larger than the component along the external force of theoperation force, and adopts the external force without any operation asthe corrected external force when the component along the external forceof the operation force has the same direction as the external force orthe external force is smaller than the component along the externalforce of the operation force.
 3. A control method of a power assistdevice comprising: an operation part operated by an operator; anoperation force detection device that detects operation force applied onthe operation part; a robot arm that supports the operation part; adrive mechanism that drives the robot arm; an external force measurementdevice that measures force applied when the robot arm is brought intocontact with an environment, or an external force estimation device thatestimates force applied when the robot arm is brought into contact withthe environment; and a speed detection device that detects movementspeed of the operation part, or a speed estimation device that estimatesthe movement speed of the operation part, wherein a corrected externalforce is obtained based on the external obtained by the external forcemeasurement device or the external force estimation device and acomponent along the external force of the operation force detected bythe operation force detection device, the drive mechanism is controlledso as to maintain the state that the operation force and the correctedexternal force act on the operation part and the component along theexternal force of the operation force and the corrected external forceacting on the robot arm are always balanced with each other, the speedalong the external force of the operation part calculated from thecomponent along the external force of the operation force detected bythe operation force detection device is compared with the speed of theoperation part calculated from the external force obtained by theexternal force measurement device or the external force estimationdevice, the speed of the operation part calculated from the externalforce is regarded as the speed along the external force of the operationpart calculated from the component along the external force of theoperation force so as to obtain the corrected external force when thespeed along the external force of the operation part calculated from thecomponent along the external force of the operation force is opposite tothe speed of the operation part calculated from the external force andthe speed of the operation part calculated from the external force islarger than the speed along the external force of the operation partcalculated from the component along the external force of the operationforce, and the external force without any operation is adopted as thecorrected external force when the speed along the external force of theoperation part calculated from the component along the external force ofthe operation force has the same direction as the speed of the operationpart calculated from the external force or the speed of the operationpart calculated from the external force is smaller than the speed alongthe external force of the operation part calculated from the componentalong the external force of the operation force.
 4. A control method ofa power assist device comprising: an operation part operated by anoperator; an operation force detection device that detects operationforce applied on the operation part; a robot arm that supports theoperation part; a drive mechanism that drives the robot arm; an externalforce measurement device that measures force applied when the robot armis brought into contact with an environment, or an external forceestimation device that estimates force applied when the supporting partis brought into contact with the environment; and a speed detectiondevice that detects movement speed of the operation part, or a speedestimation device that estimates the movement speed of the operationpart, wherein a corrected external force is obtained based on theexternal force obtained by the external force measurement device or theexternal force estimation device and a component along the externalforce of the operation force detected by the operation force detectiondevice, the drive means is controlled so as to maintain the state thatthe operation force and the corrected external force act on theoperation part and the component along the external force of theoperation force and the corrected external force acting on the robot armare always balanced with each other, the component along the externalforce of the operation force detected by the operation force detectiondevice is compared with the external force obtained by the externalforce measurement device or the external force estimation device, theexternal force is regarded as a component of the operation force alongthe external force so as to obtain the corrected external force when thecomponent along the external force of the operation force is opposite tothe external force and the external force is larger than the componentalong the external force of the operation force, and the external forcewithout any operation is adopted as the corrected external force whenthe component along the external force of the operation force has thesame direction as the external force or the external force is smallerthan the component along the external force of the operation force.